It is known that an FFV (flexible fuel vehicle) can use alcohol or alcohol-blended gasoline as fuel. An internal combustion engine for the FFV includes an alcohol concentration sensor that determines the alcohol concentration of an employed fuel. An output value of the alcohol concentration sensor is used as a parameter for air-fuel ratio control of the internal combustion engine. This not only acquires desired torque but also ensures satisfactory emissions performance without regard to the ethanol concentration of the employed fuel.
As described above, the alcohol concentration sensor for the FFV internal combustion engine plays an important role to ensure the performance of the internal combustion engine. However, as is the case with the other sensors, there is no assurance that the alcohol concentration sensor properly functions at all times. Wire breakage, short-circuiting, sensor element deterioration, or other abnormality may occur in the alcohol concentration sensor. If, in the event of such abnormality, the output value of the alcohol concentration sensor is used to control the internal combustion engine, then an operation appropriate for the alcohol concentration of the employed fuel cannot be performed. This causes the deterioration of internal combustion engine performance such as emissions performance and fuel efficiency.
As such being the case, it is demanded that the rationality of the alcohol concentration sensor for the FFV internal combustion engine be diagnosed to verify the reliability of the output value of the alcohol concentration sensor. However, it is difficult to directly verify the accuracy of the output value of the alcohol concentration sensor. The reason is that it is not easy to accurately measure the alcohol concentration by using means other than the alcohol concentration sensor. It is quite difficult to prepare a necessary standard at the time of verification.
Under such circumstances, a technology disclosed in Japanese Patent No. 4477644 determines the difference between a target air-fuel ratio and an actual air-fuel ratio obtained when a fuel injection amount is adjusted in accordance with the target air-fuel ratio, and diagnoses, in accordance with the determined difference, whether or not the alcohol concentration sensor is abnormal. When the output value of the alcohol concentration sensor is highly reliable, the actual air-fuel ratio should be in agreement with or close to the target air-fuel ratio. Therefore, if the actual air-fuel ratio significantly differs from the target air-fuel ratio, it is probable that the reliability of the output value of the alcohol concentration sensor is low.
However, air-fuel ratio changes are brought about by various multiple factors in addition to fuel injection amount changes. Hence, even when the actual air-fuel ratio is closed to the target air-fuel ratio, it cannot always be said that the rationality of the alcohol concentration sensor is maintained. The technology described in Japanese Patent No. 4477644 cannot detect an abnormality in the alcohol concentration sensor until the abnormality decreases the reliability of the output value of the alcohol concentration sensor and causes the actual air-fuel ratio to differ from the target air-fuel ratio. In other words, an alcohol concentration sensor abnormality cannot be detected until the discrepancy between the actual air-fuel ratio and target air-fuel ratio actually degrades the emissions performance.
An alternative method for diagnosing the rationality of the alcohol concentration sensor would be to prepare two alcohol concentration sensors that generate the same output when no abnormality exists, and compare the output values of the alcohol concentration sensors. If the alcohol concentration sensors generate different output values or the difference between their output values is greater than a threshold value when the alcohol concentration of the employed fuel is changed, this method makes it possible to conclude that the alcohol concentration sensors are no longer rational.
In reality, however, the alcohol concentration of the fuel employed for the FFV internal combustion engine changes only when it is fueled. Therefore, even if an alcohol concentration sensor abnormality occurs in such a manner as to decrease the reliability of the sensor output value, such an abnormality cannot be detected until a refueling is performed later. Further, if the fuel added at the time of refueling has the same alcohol concentration as the fuel remaining in a fuel tank, the output values of the alcohol concentration sensors remain unchanged. Therefore, the alcohol concentration sensor abnormality still cannot be detected.
As explained above, the conventionally proposed technology for diagnosing the rationality of the alcohol concentration sensor is not adequately capable of diagnosing the rationality of the alcohol concentration sensor properly and easily.